An ultrasound diagnostic system has been extensively used in the medical field due to its non-invasive and non-destructive nature. The ultrasound diagnostic system is highly safe and produces no dangerous side effects such as exposure to X-rays, etc. However, the ultrasound diagnostic system suffers from inherent shortcomings of an ultrasound image such as a low signal-to-noise ratio and a limited field of view. Thus, the image registration of a CT (or MR) image onto the ultrasound image has been introduced in order to compensate for deficiencies of the ultrasound image.
Conventionally, an image based registration using feature points extracted from the ultrasound image and the CT (or MR) image has been adopted for the image registration between the ultrasound image and the CT (or MR) image. This image registration may be implemented by matching the extracted feature points. However, this image based registration for registering the ultrasound image onto the CT (or MR) image is a difficult task due to their different imaging characteristics such as resolution, image quality, viewing angle, contrast, etc.